The importance of characterizing uncertainty in controversial geoscience applications: induced seismicity associated with hydraulic fracturing for shale gas in northwest England

Fracking of the Preese Hall-1 well in 2011 induced microseismicity that was strong enough to be felt. This occurrence of ‘nuisance’ microearthquakes, unexpected at the time, was a major factor resulting in adverse public opinion against shale gas in the UK and was thus of significant political importance. Despite this, and notwithstanding the technical importance of this instance of induced seismicity for informing future shale gas projects, it has received little integrated study; contradictory results have indeed been reported in analyses that lack integration. This instance therefore provides a case study to illustrate how a small but significant multi-disciplinary geoscience dataset may be put to best use, including how best to quantify uncertainties in key parameters, which may themselves be relatively poorly quantified but whose values may significantly affect the ability to understand the occurrence of induced seismicity. The best-recorded event in this induced microearthquake sequence (at 08:12 on 2 August 2011) is thus assigned an epicentre circa British National Grid reference SD 377358, south of the Preese Hall-1 wellhead, a focal depth of ∼2.5 km, and a focal mechanism with strike 030°, dip 75°, and rake −20°, this NNE-striking nodal plane being the inferred fault plane. Like other parts of Britain, this locality exhibits high differential stress, with maximum and minimum principal stresses roughly north-south and east-west. This instance indeed fits an emerging trend of the occurrence of relatively large induced earthquakes in localities with high differential stress; such an association was predicted many years ago on the basis of experimental rock mechanics data, so observational confirmation might well have been anticipated and should thus not have been unexpected. Many steep faults, striking NNE-SSW or NE-SW, mostly Carboniferous-age normal faults, are present; the stress field is favourably oriented for their left-lateral reactivation, southward leakage of fracking fluid into one such fault having presumably caused the induced seismicity. Given the pervasive presence of similarly oriented faults, future occurrences of similar induced seismicity should be planned for; they pose a significant technical challenge to any future UK shale-gas industry.